linux-kernel - Re: [PATCH v2 1/2] livepatch: introduce shadow variable API

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Date:   Fri, 21 Jul 2017 10:59:59 +0200 (CEST)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Joe Lawrence <joe.lawrence@...hat.com>
cc:     live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jessica Yu <jeyu@...hat.com>, Jiri Kosina <jikos@...nel.org>,
        Petr Mladek <pmladek@...e.com>
Subject: Re: [PATCH v2 1/2] livepatch: introduce shadow variable API

On Thu, 20 Jul 2017, Joe Lawrence wrote:

> On 07/20/2017 10:45 AM, Miroslav Benes wrote:
> > 
> >>>>> + *
> >>>>> + * Note: allocates @new_size space for shadow variable data and copies
> >>>>> + * @new_size bytes from @new_data into the shadow varaible's own @new_data
> >>>>> + * space.  If @new_data is NULL, @new_size is still allocated, but no
> >>>>> + * copy is performed.
> >>>>
> >>>> I must say I'm not entirely happy with this. I don't know if this is what 
> >>>> Petr had in mind (I'm sure he'll get to the patch set soon). Calling 
> >>>> memcpy instead of a simple assignment in v1 seems worse. 
> >>>
> >>> This change was a bit of a experiment on my part in reaction to
> >>> adding klp_shadow_get_or_attach().
> >>>
> >>> I like the simplicity of v1's pointer assignment -- in fact, moving all
> >>> allocation responsiblity (klp_shadow meta-data and data[] area) out to
> >>> the caller is doable, though implementing klp_shadow_get_or_attach() and
> >>> and klp_shadow_detach_all() complicates matters, for example, adding an
> >>> alloc/release callback.  I originally attempted this for v2, but turned
> >>> back when the API and implementation grew complicated.  If the memcpy
> >>> and gfp_flag restrictions are too ugly, I can try revisting that
> >>> approach.  Ideas welcome :)
> >>
> >> Well, I didn't like callbacks either :). And no, I do not have a better 
> >> idea. I still need to think about it.
> > 
> > Done and I agree that memcpy approach is not so bad after all :). So I'm 
> > fine with it.
> 
> I looked at it again this morning and a "pass-your-own" allocation API
> always comes back to adding callbacks and other complications :(  In the
> end, most callers will be shadowing pointers and not entire structures,
> so I think the copy isn't too bad.

I agree.
 
> On a related note, if we keep the allocations and memcpy, how about I
> shift around the attach/get calls like so:
> 
>   __klp_shadow_attach
>     set shadow variable member values
>     memcpy
>     add to hash
> 
>   klp_shadow_attach
>     alloc new shadow var
>     lock
>     call __klp_shadow_attach with new alloc
>     unlock
> 
>   klp_shadow_get_or_attach
>     be optimistic, call klp_shadow_get (if found, return it)
>     be pessimistic, alloc new shadow var
>     lock
>       call klp_shadow_get again
>       if unlikely found
>         kfree unneeded alloc
>       else
>         call __klp_shadow_attach with new alloc
>     unlock
>     return whichever shadow var we used
> 
> This way both calls can accept gfp_flags that may sleep, with the only
> downside that klp_shadow_get_or_attach may allocate an unnecessary
> shadow variable in the unlikely case that it's found on the second
> klp_shadow_get attempt (under the lock).  No more clunky "bool lock"
> flag either. :)

Yes, this looks good.

Thanks,
Miroslav